Page:Great Neapolitan Earthquake of 1857.djvu/125

Rh the fall of others. The disintegration of the building—viz., whether it shall split up at all—depends, as we have seen, upon—


 * 1st. The direction (as to horizontal obliquity and emergence) and the velocity of the wave, directly, and the density of the materials.


 * 2nd. Upon the tenacity and bond of the materials, inversely, and upon the form and magnitude of the structure.

Both these relations being modified, by the elasticity and flexibility, of the materials.

The directions of the great fractures producing disintegration depend upon—


 * 1st. The direction (as to obliquity and emergence) of the wave; i. e., on the abnormal and emergent angles.
 * 2nd. The form of the structure as a whole, and of its several parts, or details, previous to severance.

Lastly. The fall of each separate mass, if then severed, depends upon its own moment of stability. If any such separated mass fall, it does so in the direction of its least. horizontal dimension: or as for any practical purpose of seismic observation, the moment of inertia due to the oversetting force must lie in this direction (for otherwise the mass falls by twisting), we may limit the consideration to that case.

If $$t$$ be the thickness of the bed joint of masonry upon which the mass overturns, cut by a vertical plane passing through the centre of gravity of the mass and the line of Rh